Well, three ways, all of them simple, one almost done before, one alluded to this year, one yet to happen in reality or hypothetically.
1. A time machine so we can let Mr. Kennedy follow through on his plan to cash in at the next Wrestlemania, guaranteeing his spot a year in advance, starting endless storylines as every new champion develops friction with him. Every win he picked up would have validated his main event at Mania, every loss would be huge for whoever beat him. And think of the anticipation! People would have been posting thread after thread about how they love or hate the idea on here.
2. Cash it in before or after the main event the same night it is won. A face would add themselves to the match (though that perhaps cheapens or messes with the build they've been working on for the one on one feud), a heel would cash in afterwards, destroying the physically drained winner and providing an ever-lasting final image of that Wrestlemania.
3. A face wrestler who isn't quite at main event level cashes in to face the champion between Backlash and Royal Rumble and LOSES in a 15-20 minute match. ROH fans will know how well a loss can make a wrestler thanks to Tyler Black and Nigel McGuinness. For those not familiar, let me talk you through it:
The face challenges the champion in advance, heel or face, it works equally well. You do the standard build, the announcers hype that the guy either isn't ready, or has no chance, or that he has the chance of a lifetime ahead of him. Tag matches, promo encounters, hell, even a contract signing if needs-be for the build.
Then in the match itself, just have the guy kick out of nearly everything and hit all his big moves. Let him hang with the champion for a long time, showing a side of himself he hasn't before, and creating all the hype in the world for himself, but ultimately lose.
Just handing the title to someone not ready will not make them, because it's seen as a fluke win. Swagger is not a made man yet. But if you put them in the position where people believe he might actually pull it off then it makes people go "hey, this guy's got something" and they buy into it when they threaten for the title in the future. They also earn the respect of the champion and get a handshake from a face, or a handshake and then an attack from a heel. Both give them a further rub and set up more future angles.
PLUS it means we aren't positive the winner is going to win the title. RVD is the only guy who gave advance warning of his cash-in (besides Kennedy, but that didn't work out), and if it wasn't at One Night Stand, people wouldn't have been sure if he was going to win or not. If every single guy who cashes in wins then it takes some suspense out of it. This method of having the guy cash in, give it his all and lose, keeps that suspense.
Tell me this wouldn't be a great angle for the likes of a Kofi Kingston or John Morrison.